For the purposes of this disclosure, a milling operation in a machine shop refers to the practice of securing, a work piece in a vise that is clamped to a table that can be moved with three degrees of freedom against a rotating toolbit (cutter) that cuts away portions of the work piece in accordance with a require plan. The rotational axis of the toolbit is maintained stationary and is generally vertical. The various operations that can be performed with this arrangement include drilling holes and cutting linear slots and chamfering the edges of holes and slots. Common practice is to machine the work piece having complex surfaces using a toolbit designed for the specific operation. For example, the operation of drilling a hole requires a drill bit. moving the work piece in a linear direction so as to cut a straight groove or form a flat surface on the part requires an end mill, having a cylindrical shaft with radially extending cutting flutes.
Most jobs involve various operations in combination so that any given job usually requires several different toolbits. This necessitates performing one operation with one toolbit and then changing the toolbit to peform another operation. As a result of this situation, the present state of the art equipment has arrangements such as "quick-change" chucks, brakes, automated too changers, etc. for reducing the accumulated "down time" of the machine required for changing the tool.
A very frequent requirement for many designs is the requirement to chamfer the edge of a slot that has been cut by an end mill. Milling the slot in a first operation, changing the tool bit from an end mill to a chamfering tool and then milling the chamfer takes at least twice the time of simply milling a slot (without chamfer). A milled slot leaves a corner that has to be deburred so that, even when no cramfer is required, the careful workman will go back over the work piece and "break the edge" with a debuning tool. Another factor that adds time and expense to a milling operation involving machining an opening in a group of "boards" such as in the manufacturing of frames for a printed circuit operation is the requirement to lubricate the part with an appropriate coolant. The coolant is typically a water base lubricant and a popular method of applying the lubricant is to direct a fine spray on the surface of the part. This method is inefficient from the standpoint that the lubricant lands on a broad surface of the part far removed from the exact location where the lubricant is needed. This location is on the cutter blade about 0.050 inches from where the chip separates from the metal and where the chip rubs against the cutter surface thereby generating excessive heat.
Yet another inconvenience associated with the job of milling an opening in a group of plates is that the cutting edge of the tool bit wears and the toolbit flexes so that sources of variations are introduced which result in loss of precision in machining the workpiece.